Strange Shadow on mesh?
As @Carlo suggest, the straight line is not a shadows, but the texture instead. 

By changing size in the Mapping Node, you are distorting the image and the UV Map doesn't match as expected.


If you toggle muteM Mapping Node, your texture is correct.



 

In the Blender Guru Example, the Mapping Node is applied to the Noise Texture to achieve a "scratchy" effect.